The England international left-back's current contract with the Blues runs out at the end of the season, and talks over a new deal have been going on.
"I'm very optimistic that it will be dealt with before that [January]," said Di Matteo. "He has become a legend - he is now Chelsea through and through.
"But I wouldn't say it's just happened recently. He's been here many years and has contributed a lot to this club.
"He's a fantastic pro, which I'm not sure whether you know."
Di Matteo then hailed Cole's professionalism and commitment.
"Somebody that's the first one out in training, one of the first into the building with JT [John Terry], trains every day like a professional, reliable, somebody that you could go to war with," he said.
"And he's a very quiet guy as well. He's very private at his own stuff. He's one of the best left-backs in the world."
Cole joined Chelsea from Arsenal in 2006.